Recently, in refrigerating machines and the like, hydrofluorocarbons (HFCs) having ozone depletion potentials of zero have been used as refrigerants. However, the European Union issued a directive in 2006 to ban the use of refrigerants having global warming potentials (GWPs) of more than 150 as refrigerants used for automobile air conditioning. Based on the European Union directive, the regulations for stationary air conditioners and the like using HFC refrigerants may also be revised in the future. Hence, alternative refrigerants have been studied.
Candidates for the alternative refrigerants are said to be refrigerants including hydrofluoroolefins such as 1,3,3,3-tetrafluoropropene (HFO-1234ze) and 2,3,3,3-tetrafluoropropene (HFO-1234yf) (Japanese Patent Application Publication No. 2012-31239).
However, a hydrofluoroolefin refrigerant having unsaturated bonds generate acids by oxidative decomposition. Further, the generated acid may decrease the stability of a refrigerating machine oil composition. Meanwhile, hydrofluoroolefin refrigerants have very high miscibilities with refrigerating machine oil compositions. Hence, the resulting refrigerant solutions have quite low viscosities and insufficient lubricities, which are indicated by friction, wear, or the like. Hence, a refrigerating machine oil composition used together with a hydrofluoroolefin refrigerant is required to have an excellent lubricity. Although an additive such as an extreme pressure agent or an anti-wear agent is normally added to enhance the lubricity of a refrigerating machine oil composition, the refrigerating machine oil composition is required to exhibit an excellent response to such additives (hereinafter, additive response), too.
International Publication No. WO2013/125528 discloses a refrigerating machine oil composition containing: at least one ester additive selected from tetraesters of pentaerythritol and one selected from fatty acids having 5 to 18 carbon atoms and hexaesters of dipentaerythritol and one selected from fatty acids having 5 to 18 carbon atoms; and a base oil that is an ester other than the ester additive. The content of the ester additive is 2 to 20% by mass based on a total amount of the refrigerating machine oil composition, and the kinematic viscosity of the refrigerating machine oil composition at 40° C. is 3 to 500 mm2/s.
International Publication No. WO2013/125528 states that the fatty acid used in the ester additive more preferably has 6 to 9 carbon atoms, but specifically discloses only a tetraester of pentaerythritol and a fatty acid having 8 carbon atoms, and a hexaester of dipentaerythritol and a fatty acid having 6 carbon atoms.